Although increasing recognition is being given to the psychologic needs of children in hospital, many gaps still exist in our knowledge about this topic. The present paper is concerned with one of these, namely, the significance of maternal deprivation occurring in the early months of life. Under standardized conditions of observation and analysis of data, 76 infants less than 12 months of age were observed while undergoing short-term hospitalization. Observations were focused on three points: the period immediately after admission, the period preceding discharge, and the period after return home. From the data thus obtained two main syndromes emerged, each linked to a certain age range, with a dividing point at approximately 7 months. Those above this age showed essentially the same type of behavior as that described in other studies of the preschool group, namely, considerable upset when admitted to hospital and a period of disturbance after return home, both centering around the need for the physical presence of the mother. In the group less than 7 months of age, on the other hand, separation from the mother evoked no observable disturbance and, instead, an immediate adjustment to the new environment and the people in it was found. On return home a marked change of behavior occurred in most of these younger infants, but it was generally of very brief duration. On the basis of the data presented it is suggested that the critical period, when separation from the mother is experienced as a traumatic event, does not commence until after the middle of the first year of life, and that consequently in those cases where there is a choice, hospitalization should be arranged to occur before the crucial age is reached.